U.S. patent number 4,567,362 [Application Number 06/504,602] was granted by the patent office on 1986-01-28 for process and apparatus for the focusing of a beam of light on an object.
This patent grant is currently assigned to GRETAG Aktiengesellschaft. Invention is credited to Rino E. Kunz.
United States Patent |
4,567,362 |
Kunz |
January 28, 1986 |
Process and apparatus for the focusing of a beam of light on an
object
Abstract
A laser beam impacts an object through a focusing lens. The beam
spot produced on the object is reproduced by means of an optical
assembly of three measuring diaphragms in the form of a stripe of a
width varying as a function of the state of focusing, with the
measuring diaphragms being mutually offset with respect to the
ideal focusing point. The light passed by the measuring diaphragms
impacts three photoreceivers and the measuring signals produced by
them are processed in an evaluating stage to produce a correction
signal which then is used for the automatic setting of the focus by
means of a control device. The system requires no mechanically
oscillating parts, is simple in its configuration and suitable for
pulsed laser systems.
Inventors: |
Kunz; Rino E. (Steinmaur,
CH) |
Assignee: |
GRETAG Aktiengesellschaft
(Regensdorf, CH)
|
Family
ID: |
4266399 |
Appl.
No.: |
06/504,602 |
Filed: |
June 15, 1983 |
Foreign Application Priority Data
|
|
|
|
|
Jun 25, 1982 [CH] |
|
|
3923/82 |
|
Current U.S.
Class: |
250/201.4;
356/4.05 |
Current CPC
Class: |
B23K
26/04 (20130101) |
Current International
Class: |
B23K
26/04 (20060101); G01J 001/20 () |
Field of
Search: |
;250/201-204,227,229,237G ;354/403,406,407 ;350/448,449,3.6
;356/1,4 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Optical Profilometer for Monitoring Surface Contours of Si Power
Devices", H. P. Kleinknecht and H. Meier, Laboratories RCA Ltd.,
pp. 266-273..
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Gatto; J.
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Claims
What is claimed is:
1. A process for focusing a light beam, in particular a laser beam,
on an object, comprising the steps of producing by means of an
optical system at least two images of a beam spot formed by the
light beam on the object, providing a measuring diaphragm
associated with each of said at least two images, said diaphragm
being located in different positions relative to the desired
position of the image of the beam spot in the case of optimum
focusing, evaluating the intensities of the light not blocked out
by the measuring diaphragms as a measure of focusing, and adjusting
the focusing of the light beam in response to the evaluation, said
at least two images of the beam spot being produced by means of a
diffraction grating.
2. A process according to claim 1 wherein the beam spot is
reproduced on the measuring diaphragms in the form of a strip of
light of a width varying as a function of the state of
focusing.
3. A process according to claim 2 wherein the measuring diaphragms
have an essentially V-shaped orifice.
4. A process according to claim 2 wherein the measuring diaphragms
have an essentially rhomboidal diaphragm orifice, one diagonal
whereof is located essentially in a center axis of the strip of
light.
5. A process according to claim 2 wherein the measuring diaphragms
have a diaphragm orifice bordered by nonlinear edges.
6. A process according to claim 1 wherein three images of the beam
spot are produced, with one of the measuring diaphragms being
located in front, one of the measuring diaphragms essentially in,
and one of the measuring diaphragms behind the desired position of
the image of the beam spot when optimally focused.
7. Apparatus for the focusing of a beam of light, in particular a
laser beam, on an object impacted by said beam, comprising an
optical system which produces at least two images of a beam spot
formed by the beam on the object, the optical system comprising a
diffraction grating, two measuring diaphragms each associated with
one image, said diaphragms being arranged in different positions
relative to the desired positions of the images of the beam spot in
the case of optimum focusing, and opto-electrical means for
measuring the light not blocked out by the measuring diaphragms and
producing a signal characteristic of the state of focusing.
8. Apparatus according to claim 7 wherein the optical system
reproduces the beam spot in the form of a strip of light with a
width varying as a function of the state of focusing, onto the
measuring diaphragms.
9. Apparatus according to claim 7 wherein the measuring diaphragms
have essentially triangular orifices.
10. Apparatus according to claim 7 including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
11. Apparatus according to claim 8 including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
12. Apparatus according to claim 9 including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
13. Apparatus according to claim 7 wherein the measuring diaphragms
are formed by light inlet surfaces of the opto-electrical means
essentially arranged in one plane of light conducting elements, in
particular light conducting fibers.
14. Apparatus according to claim 8 wherein the measuring diaphragms
are formed by light inlet surfaces of the opto-electrical means
essentially arranged in one plane of light conducting elements, in
particular light conducting fibers.
15. Apparatus according to claim 9 wherein the measuring diaphragms
are formed by light inlet surfaces of the opto-electrical means
essentially arranged in one plane of light conducting elements, in
particular light conducting fibers.
16. Apparatus according to claim 10 wherein the measuring
diaphragms are formed by light inlet surfaces of the
opto-electrical means essentially arranged in one plane of light
conducting elements, in particular light conducting fibers.
17. Apparatus according to claim 7 wherein three measuring
diaphragms are present, with one of the measuring diaphragms
disposed in front, one of the diaphragms essentially in and one
diaphragm behind the desired position of the image of the beam spot
with an optimally focused beam of light.
18. Apparatus according to claim 8 wherein three measuring
diaphragms are present, with one of the measuring diaphragms
disposed in front, one of the diaphragms essentially in and one
diaphragm behind the desired position of the image of the beam spot
with an optimally focused beam of light.
19. Apparatus according to claim 9 wherein three measuring
diaphragms are present, with one of the measuring diaphragms
disposed in front, one of the diaphragms essentially in and one
diaphragm behind the desired position of the image of the beam spot
with an optimally focused beam of light.
20. Apparatus according to claim 10 wherein three measuring
diaphragms are present, with one of the measuring diaphragms
disposed in front, one of the diaphragms essentially in and one
diaphragm behind the desired position of the image of the beam spot
with an optimally focused beam of light.
21. Apparatus according to claim 13 wherein three measuring
diaphragms are present, with one of the measuring diaphragms
disposed in front, one of the diaphragms essentially in and one
diaphragm behind the desired position of the image of the beam spot
with an optimally focused beam of light.
22. Apparatus for focusing a beam of light on a workpiece
comprising:
optical means for producing at least two shaped images of a beam
spot formed by the beam of light on the workpiece;
a first measuring diaphragm having a shaped opening therethrough,
the first measuring diaphragm being associated with one of the
images and being positioned in spaced relation in one direction
relative to a desired focal position of the one image, the shape of
the opening cooperating with the shape of the image to cause
variations in the intensity of light passing through the opening in
response to variations in the focus of the beam of light on the
workpiece;
a second measuring diaphragm having a shaped opening therethrough,
the second measuring diaphragm being associated with another of the
images and being positioned in spaced relation in a direction
opposite said one direction relative to a desired focal position of
the other image, the shape of the opening cooperating with the
shape of the image to cause variations in the intensity of light
passing through the opening in response to variations in the focus
of the beam of light on the workpiece;
means for evaluating the light passing through the opening in the
first and second measuring diaphragms; and
means for modifying the focus of the light beam on the workpiece in
response to said evaluating means.
23. Apparatus of claim 22 wherein said optical means comprises a
diffraction grating which shapes the image of the beam spot into at
least two elongated, relatively narrow stripes, and means for
focusing each of said stripes at the respective desired focal
positions of the image.
24. Apparatus of claim 22 wherein said optical means comprises
holographic optical means for shaping the image of the beam spot
into at least two identical images focused at the respective
desired focal positions of the image.
25. Apparatus for focusing a laser beam on a workpiece
comprising:
an optical element disposed to receive an image of a beam spot
formed by the laser beam on the workpiece, the optical element
forming at least first and second images of the beam spot each
having a unique shape differing from the shape of the beam spot on
the workpiece and varying in size in relation to the focus of the
beam spot on the workpiece;
signal generating means for receiving each of the first and second
images of the beam spot and photoelectrically generating measuring
signals in response thereto the signal generating means being
arranged such that a first and second measuring signals of equal
values are produced when the beam spot on the workpiece is at
optimum focus, one of said first and second signals being greater
than the other by an amount increasing with the amount of focus
error of the beam spot relative to optimum focus in one direction
and the other of the first and second signals being greater than
the one by an amount increasing with the amount of focus error of
the beam spot relative to optimum focus in a direction opposite
said one direction,
means for modifying the focus of the beam spot on the workpiece in
response to said first and second signals.
26. Apparatus of claim 25 wherein said optical element is a
diffraction grating producing at least two images of the beam spot
in the form of elongated stripes.
27. Apparatus of claim 25 wherein said signal generating means
includes first and second photoelectric cells disposed to receive
the respective first and second images of the beam spot and to
generate respective first and second signals related in value to
the intensity of light received thereby, and first and second
measuring diaphragms disposed between said optical element and the
first and second photoelectric cells, respectively, the diaphragms
having openings shaped to vary the amount of light from the images
which is received by the respective photoelectric cells as a
function of the focusing error of the beam spot on the
workpiece.
28. Apparatus according to claim 7 wherein the measuring diaphragms
have essentially rhomboidal diaphragm orifices.
29. A process for focusing a light beam, in particular a laser
beam, on an object, comprising the steps of producing by means of
an optical system at least two images of a beam spot formed by the
light beam on the object, providing a measuring diaphragm
associated with each of said at least two images, said diaphragms
being located in different positions relative to the desired
position of the image of the beam spot in the case of optimum
focusing, evaluating the intensities of the light not blocked out
by the measuring diaphragms as a measure of focusing, and adjusting
the focusing of the light beam in response to the evaluation, said
at least two images of the beam spot being produced by means of a
holographic optical element.
30. A process according to claim 29, wherein the beam spot is
reproduced on the measuring diaphragms in the form of a strip of
light of a width varying as a function of the state of
focusing.
31. A process according to claim 29, wherein the measuring
diaphragms have an essentially V-shaped orifice.
32. A process according to claim 29, wherein the measuring
diaphragms have an essentially rhomboidal diagram orifice, one
diagonal whereof is located essentially in a center axis of the
strip of light.
33. A process according to claim 29, wherein the measuring
diaphragms have a diaphragm orifice bordered by non-linear
edges.
34. Apparatus for the focusing of a beam of light, in particular a
laser beam, on an object impacted by said beam, comprising an
optical system which produces at least two images of a beam spot
formed by the beam on the object, the optical system comprising a
holographic optical element, two measuring diaphragms each
associated with one image, said diaphragms being arranged in
different positions relative to the desired positions of the images
of the beam spot in the case of optimum focusing, and
opto-electrical means for measuring the light not blocked out by
the measuring diaphragms and producing a signal characteristic of
the state of focusing.
35. Apparatus according to claim 34, wherein the optical system
reproduces the beam spot in the form of a strip of light with a
width varying as a function of the state of focusing, onto the
measuring diaphragms.
36. Apparatus according to claim 34, wherein the measuring
diaphragms have essentially triangular orifices.
37. Apparatus according to claim 34, wherein the measuring
diaphragms have essentially rhomboidal diaphragm orifices.
38. Apparatus according to claim 34, including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
39. Apparatus according to claim 35, including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
40. Apparatus according to claim 36, including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
41. Apparatus according to claim 37, including control means for
automatically optimizing the focusing of the beam on the object as
a function of the characteristic signal produced by the
opto-electric means.
42. Apparatus according to claim 34, wherein the measuring
diaphragms are formed by light inlet surfaces of the
opto-electrical means essentially arranged in one plane of light
conducting elements, in particular light conducting fibers.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a process and an apparatus for
focusing a light beam, in particular a laser beam, on an object
impacted by said laser beam.
The processing of workpieces by means of laser beams is gaining
steadily in importance. One of the most significant problems in
this context is that of focusing the laser beam on the surface to
be worked in an optimum manner while continuously maintaining this
optimum focus setting and constantly regulating it.
Various methods are known for focusing and automatic controlling
focus setting. One of these is described for example in German Pat.
No. 2,034,341 and U.S. Pat. No. 3,689,159. In the method described
in these patents, the beam spot produced on the object by the
working laser beam or an auxiliary laser is reproduced on a
measuring diaphragm oscillating in the direction of the optical
axis, with the neutral position of the measuring diaphragm
coinciding with the image plane of the beam spot in the case of
ideal focusing. The light passed by the diaphragm is guided to a
photoreceiver. With optimum focusing, the image of the beamspot is
located exactly in the center of the oscillating path of the
measuring diaphragm and the variation in time of the measuring
signal produced by the photoreceiver yields a symmetrical,
essentially sinusoidal curve. In case of deviations from the
optimum state of focusing the image of the beam spot is displaced
from the center of the oscillating path of the measuring diaphragm
and the measuring signal becomes correspondingly asymmetrical. This
asymmetry is then evaluated by means of a phase sensitive detector
for the setting and continuous regulation of focusing.
A further system described, for example, in German Pat. No.
2,453,364 uses in place of an oscillating measuring diaphragm an
oscillating focusing lens.
These known systems work very well in principle, but have a number
of disadvantages. They require, for example, a mechanically
oscillating measuring or objective lens with all the inherent
disadvantages of mechanically moving parts, and they are relatively
demanding with respect to electronics. Furthermore, they are
intended primarily for continuous lasers and are less suitable for
pulsed lasers. Finally, they have no particular sensitivity in the
vicinity of the optimum focusing point.
French Pat. No. 94,871 sescribes a laser scanning system for
surface reliefs operating essentially without moving parts. In
place of one moving measuring diaphragm the beam spot is reproduced
simultaneously on two stationary measuring diaphragms. The two
measuring diaphragms are located in front of and behind the image
plane of the beam spot in the case of ideal focusing. The light
passed by the measuring diaphragms impacts on two photoreceivers,
the output signals of which represent a measure of the state of
focusing or the dimension in depth of the relief scanned.
In this known system the light emitted by the beam spot and finally
impacting the two photoreceivers is divided into two separate
measuring beam paths by means of two beam splitter/mirrors or a
mask in a spatially, entirely inhomogeneous manner. Mechanical and
thermal effects and those caused by electrical/magnetic fields and
refractory index variations, may affect the two measuring beam
paths with different intensity, thereby causing significant
measuring errors. The elimination or compensation of harmful
effects, on the other hand, is very difficult, laborious and
expensive.
U.S. Pat. No. 3,614,456 shows a further focusing system, which
operates in a manner similar to that of French Pat. No. 94,871, but
with a single measuring diaphragm. This system reacts very
sensitively to interference, is difficult to adjust and is
therefore not suitable for practical use in systems with high
precision requirements.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
The present invention is intended to improve a process and an
apparatus of the above described type, so that, while using the
lowest possible number of optical components, a spatially
homogeneous distribution of the measuring beam path is obtained,
whereby each partial measuring beam experiences the same fate
(identical conditions concerning angle of deflection, polarization,
temperature of the photoreceivers, etc.). Pulse operation should be
possible and the highest possible sensitivity in the vicinity of
optimum focusing should be attained.
The foregoing and other objects are achieved as set forth in the
appended independent claims. Preferred variants of embodiment and
further developments are set forth in the dependent claims.
More specifically, at least two images of a beam spot formed on a
workpiece by a light beam are produced by an optical system. A
measuring diaphragm is associated with each of the images, and each
diaphragm is positioned in a different location relative to the
desired position of the beam spot image in the case of optimum
focusing. The light passed by the measuring diaphragms is evaluated
as a measure of focusing, and the focus of the light beam on the
workpiece is adjusted in response to the evaluation.
The optical system that forms the at least two images preferably
includes a diffraction grating or an equivalent holographic optical
element. The measuring diaphragms are shaped so that the light
passed varies in intensity with variations in focusing.
The invention will become more apparent from the drawings attached
hereto. In the drawings:
FIG. 1 shows a schematic view of a first embodiment of the
apparatus according to the invention;
FIG. 2 shows a section of the apparatus of FIG. 1 illustrating in
greater detail certain measuring parts of the invention;
FIGS. 3a-3d are diagrams illustrating the measuring diaphragms and
a response curve for purposes of explaining the mode of operation
of the FIG. 1 embodiment;
FIGS. 4a-4c illustrate three examples of further possible
configurations of measuring diaphragms;
FIGS. 5a and 5b illustrate alternative embodiment of a measuring
diaphragm;
FIG. 6 illustrates a second embodiment of an apparatus according to
the invention;
FIG. 7 illustrates an examplary measuring diaphragm and beam spot
image produced at the diaphragm in accordance with the FIG. 6
embodiment; and,
FIG. 8 is a schematic diagram illustrating the preparation of a
holographic element for the examplary embodiment according to FIG.
6.
DETAILED DESCRIPTION
The laser processing apparatus schematically shown in FIG. 1 is
similar in its fundamental configuration to the known state of the
art, as indicated for example by the aforementioned U.S. Pat. No.
3,689,159, so that the description following hereinbelow may be
restricted to the parts that are essential for the preferred form
of the present invention.
The apparatus shown includes a laser source 1, projecting a laser
beam L through a semipermeable mirror 2 and a focusing lens 3 onto
a workpiece or object 4 to be processed, thereby producing a beam
spot LF on said workpiece of a size varying with the focusing
state. The reflected laser light LR from this beam spot LF arrives
through the focusing lens 3 and the semipermeable mirror 2 onto an
optical assembly comprising a diffraction grating (ruled grating) 5
and a cylinder lens 6, which splits the beam path into three parts
and reproduces the beam spot LF simultaneously on three measuring
diaphragms Ma, Mb and Mc, in the form of three lines or stripes Sa,
Sb and Sc, which in the present case are vertical. The measuring
diaphragms are offset laterally and in the direction of the optical
axis A (Z axis) with respect to each other. The light passing
through the measuring diaphragms, i.e. not blocked out by them, is
then directed by means of three further cylindric or more generally
anamorphic cylinder lenses 7a, 7b and 7c onto three photoreceivers
8a, 8b and 8c and is measured by them. The electrical signals Ia,
Ib and Ic corresponding to the light measured depend on the state
of focusing and are conducted to an evaluating stage 9, which
produces from said signals a correction signal Ik characteristic of
the existing state of focusing. This signal is then used by a
control device consisting of a servo-drive circuit 11 and a
servo-drive motor 12, to displace the workpiece 4 in the direction
of the z-axis relative to the focusing lens 3 until the optimum
focus setting is attained. Obviously, it is also possible to
displace the focusing lens relative to the workpiece.
FIG. 2 shows relevant parts of the apparatus of FIG. 1 in a plan
view, with the cylinder lenses 7a-7c eliminated for the sake of
simplicity. As it is seen, the three measuring diaphragms, aside
from their lateral offset, are mutually offset in the z direction,
i.e. in the direction of the optical axis A of the system. The
measuring diaphragm Ma is located exactly in the image or focal
plane FP of the system, i.e. in the plane in which the image of the
beam spot LF is produced when the laser beam L is focused exactly
or optimally on the object 4. The position of this image plane FP
shall be designated hereinafter as the desired position. The
measuring diaphragm Mb is located slightly behind the measuring
diaphragm Ma or the desired position FP, and the measuring
diaphragm Mc is located in front of the desired position FP,
symmetrically with respect to the diaphragm Mb, such that the axial
distance between diaphragm Ma-Mb is equal to the axial distance
between diaphragm Ma-Mc. However, situations may be encountered
where asymmetrically disposed diaphragms may be advantageous, e.g.
to compensate for other asymmetries in the system, for example due
to aberrations.
The mode of operation of the three measuring diaphragms Ma-Mc is
shown in FIGS. 3a-3d. Each measuring diaphragm is equipped with a
V-shaped diaphragm orifice 13, arranged symmetrically with respect
to the associated images Sa, Sb, Sc of the beam spot LF. The
representations in FIGS. 3a-3c corresponds to the conditions of
optimum focusing. Under this condition, the image Sa of the beam
spot LF is produced exactly at the axial location of the diaphragm
Ma and is ideally a narrow, line-shaped image, so that practically
none of this image is blocked by the measuring diaphragm Ma. The
corresponding measuring signal Ia of the photoreceiver 8a is thus a
maximum under this desired condition. The two other measuring
diaphragms 8b and 8c are outside the exact image plane, so that the
images Sb and Sc of the beam spot LF appear on them in the form of
relatively wide stripes. Because of the configuration of the
diaphragm orifices 13, with increasing widths of the images a
greater part of the light is blocked and the measuring signal Ib or
Ic of the associated photoreceiver becomes correspondingly
smaller.
The situation is similar when the laser beam L is not focused
accurately on the workpiece 4. In this case, the three images Sa-Sc
of the beam spot LF do not appear exactly in the desired position
FP but either in front or behind. Correspondingly, the measuring
signals Ia-Ic of the three photoreceivers 8a-8c will assume an
intermediate value between their possible extreme values. In FIG.
3d, the variations of the three measuring signals Ia-Ic are shown
as functions of the displacement of the actual image plane of the
three beam spot images with respect to the desired position FP and
thus simultaneously as functions of the state of focusing. The
abscissa points Pc, Pa and Pb mark the positions of the three
measuring diaphragms Mc, Ma and Mb.
It follows from the above that conclusions may be drawn from the
size of the three measuring signals Ia-Ic concerning the prevailing
state of focusing. If Ic is larger than Ib, the object 4 is too far
from the focusing lens 3. If, conversely, Ib is larger than Ic, the
object 4 is too close to the lens 3. The point of optimum focusing
is attained when the two measuring signals Mb and Mc are of equal
size, i.e. the measuring signal Ma is at a maximum. It is further
obvious that in principle two measuring diaphragms alone are
sufficient to determine the deviation from the optimum focusing
point with respect to direction and amount. The use of a third
measuring diaphragm in the desired position yields, however, an
improvement of the characteristic curve, together with compensation
of certain interference values, such a variation of the beam
intensity or the surface properties of the object 4.
The correction signal Ik characteristic of the state of focusing
may be calculated in the evaluation unit 9 approximately by the
formula Ik=Ib-Ic when two measuring diaphragms Mb and Mc are used
and approximately by the formula Ik=(Ib-Ic)/Ia when all three
measuring diaphragms Ma-Mc are used.
As shown in FIG. 3d, the relationship between the measuring signal
and the state of focusing, when 3 measuring diaphragms are used
according to FIGS. 3a-3c, is essentially linear in the vicinity of
the point of optimum focusing. The correction signal Ik behaves in
a similar manner, while its slope and thus the sensitivity of the
system in the vicinity of the focus point may be adapted by an
appropriate configuration of the measuring diaphragms to actual
requirements. Naturally, nonlinear characteristic curves may be
obtained by using suitable forms of the measuring diaphragms and of
optical imaging (for example spherical lenses).
In FIGS. 4a-4c, three further possible configurations of the
measuring diaphragms are shown. The measuring diaphragm Md
according to FIG. 4a has a rhomboidal diaphragm orifice 14 and
yields a characteristic curve twice as steep as the V diaphragms
described above. The same is true for the measuring diaphragm Me in
FIG. 4b, with the difference that it has an uncontrolled dead zone
for a certain minimum deviation from optimum focusing. The
measuring diaphragm Mf according to FIG. 4c finally has a diaphragm
orifice 16 bordered by convex edges and yields a curved
characteristic line which is particularly steep in the vicinity of
the optimum focal point.
The description set forth immediately hereinabove relates to
different characteristics obtained with various shapes of
transmitting measuring diaphragms. It is evident that reflecting
measuring diaphragms are also suitable for the invention. Measuring
diaphragms may further be realized by means of an appropriately
shaped inlet window of the photoreceivers or a suitable
configuration or arrangement of the photoreceivers themselves or by
means of certain light conductor arrangements or the like. The
latter is shown for example in FIGS. 5a and 5b, where a bundle of
light conductors 20 is effected so that the light inlet surfaces of
all of the individual conductors are in one plane and have for
example the configuration shown in FIG. 5b. This arrangement of
photoconductors is approximately equivalent to the measuring
diaphragms according to FIG. 4a.
It is further understood that the splitting of the reflected laser
light LR into three beam paths must not necessarily be effected by
means of a diffraction grating. Naturally, other means such as beam
splitters or the like, may also be used. Furthermore, it is not
absolutely necessary to reproduce the beam spot LF in the form of a
line or a stripe, as naturally any other geometric shape is
possible. Imaging as a line, however, proved to be particularly
appropriate in view of the control behavior of the entire
system.
An especially advantageous and elegant mode of imaging the beam
spot LF may involve the use of a hologram or a holographic optical
element, respectively. As a hologram is able to perform several
optical functions simultaneously, this makes an especially compact
configuration of the apparatus possible. FIG. 6 shows an examplary
embodiment of the apparatus according to the invention equipped
with two photoreceivers only.
The light LR reflected by the beam spot arrives through the
focusing lens 3, not shown, and and the mirror 2 onto a holographic
optical element HOE. The latter modulates the light and thereby
produces two line or stripe shaped images of the beam spot. These
images may be located in one plane, whereupon it would be necessary
to operate, as in FIGS. 1 and 2, with axially offset measuring
diaphragms. In the example according to FIG. 6, the two images are
mutually offset and the two measuring diaphragms Mg and Mh are
located in one plane between the two image planes of the beam spot.
The light passed by the two measuring diaphragms impacts the two
photoreceivers 8g and 8h, the measuring signals Ig and Ih of which
are then further processed in keeping with the aforedescribed
process.
With the aid of the holographic optical element or hologram HOE the
geometric form of the images of the beam spot may further be
affected or selected so that the measuring diaphragms may have a
very simple configuration or may be eliminated altogether. This is
shown as an example in FIG. 7, wherein the image LFB of the beam
spot is approximately star-shaped and the diaphragm orifice 21 of
the measuring diaphragm Mk is circular.
The production of the hologram may be effected by conventional
methods and shall be described hereinafter purely as an example for
a hologram producing two images mutually offset in the axial
direction, (and transversely to it), in accordance with FIG. 8.
To produce a hologram with two spatially arranged image points P+
and P-, it is merely necessary to record the interference pattern
of the spherical waves u+ and u- emanating from the two points P+
and P- with a planar reference wave Ur on a plate H coated with a
photosensitive recording material. The two spherical waves u+ and
u- are produced by means of two lenses 31 and 32 with the points P+
and P- as the focal points, form a laser beam 33. Similarly, the
planar reference wave Ur is derived from a laser beam 33 by means
of a beam expander 34.
The photographically developed and fixed hologram plate H may now
be inserted as the holographic optical element HOE in the apparatus
according to FIG. 6 and then yields, when illuminated from the
reverse side (in FIG. 8 from the left), the two desired images of
the beam spot.
The principles, preferred embodiments and modes of operation of the
present invention have been described in the foregoing
specification. The invention which is intended to be protected
herein, however, is not to be construed as limited to the
particular forms disclosed, since these are to be regarded as
illustrative rather than restrictive. Moreover, variations and
changes may be made by those skilled in the art without departing
from the spirit of the present invention.
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